1. Field of the Invention
This invention relates to an image sensing system using an illuminating device which is disposed either within or without the system.
2. Description of the Prior Art
Previously, photographing in a dark place with the conventional camera of the type using a silver halide film has been carried out using an artificial illumination device such as a lamp or a flash device as an auxiliary illuminating means, because there is an increasable limit to the brightness of the photo-taking optical system or to the sensitivity of the film. The same situation occurs with an image sensing system of the kind using a CCD or a camera tube. However, artificial illuminating devices are capable of emitting only a relatively small quantity of light, the reachable distance of which is limited. With the camera of the conventional silver halide film type, an underexposure has been inevitable for an object located beyond the reach of the artificial illumination light. The same may be said of the image sensing system of the above-stated kind. In the case of the latter, the requirement for exposure accuracy is much more severe than the camera of the silver halide film type. A slight degree of underexposure results in a salient degradation of image quality. Video cameras and the like have employed an automatic gain control circuit (hereinafter will be called AGC for short) with a negative feedback loop for accurate exposure control. With the AGC employed, the average level of the signal transmitted to signal processing and recording circuits can be kept nearly constant irrespective of to whether the output of an image sensor is large or small. However, in the event that the exposure condition existing at the time of photographing becomes entirely different from the condition which existed at the time of the light measurement carried out prior to photographing, as in the case of flash photography, the above-stated arrangement of the prior art has been incapable of coping with such a change.